Load Hamlc helpers from GlobalContext

Instead of including hamlcoffee helpers manually for server side templates
(where we have to manage the compilation pipeline) just expect them to be
injected into the global context asset. Like so (if using the default,
context path):

# app/javascripts/templates/context.js.<handler>.<handler>...
//= require hamlcoffee

This simplifies the HamlCoffeeAssets module by allowing us to delete the
helper related code. All helpers come in through the global context asset
Monkey patch ActionView::Template so it doesn't swallow ExecJS exceptions
Update readme to reflect change to global context helper inclusion
HamlCoffeeAssets#helpers returns global context (for back compatibility)
Merge pull request #91 from thedeeno/asset-dependencies

Load Hamlc helpers from GlobalContext